1. Field of the Invention
The present invention relates to an image sensing apparatus having a function of removing an alien substance sticking to an optical element arranged in front of an image sensing element, and a control method thereof.
2. Description of the Related Art
In a lens interchangeable digital camera, when the lens is detached from the camera body, dust floating in air may enter the body. A camera incorporates various kinds of mechanical units that mechanically operate, including, e.g., a shutter mechanism. When these mechanical units operate, dust such as metal pieces may be produced in the camera body. When such an alien substance, i.e., dust sticks to the surface of the image sensing element included in the image sensing unit of the digital camera, the alien substance is seen in a taken image as a shadow, making the image quality poor. In a camera using a silver halide film, the film is fed every time an image is sensed. Hence, no alien substance is sensed continuously at the same position of images. However, a digital camera has no film movement in every photographic operation. Hence, an alien substance is sensed continuously at the same position of images.
Japanese Patent Laid-Open No. 2002-204379 proposes a technique of vibrating a cover glass (dust protecting glass) for protecting the image sensing element of a conventional lens interchangeable digital camera, thereby removing dust sticking to the glass. Such alien substance removal by vibration operation increases the release time lag and reduces the continuous shot speed. To solve this problem, Japanese Patent Laid-Open No. 2004-242158 proposes a method of executing alien substance removal only before photographing of the first frame and inhibiting alien substance removal from the second frame in the continuous shot mode.
In both cases, the alien substance removal is executed before photographing. Hence, a time lag is generated from pressing of the release button to actual execution of photographing. Alien substance removal may be done in, e.g., powering on the camera. Even in this case, the same problem occurs if the release button is operated at the time of alien substance removal, and a good shutter release opportunity may be missed.